In a direct injection internal combustion engine, a fuel injector is provided to deliver a charge of atomised fuel into a combustion chamber prior to ignition. Typically, the fuel injector is mounted in a cylinder head of an engine with respect to the combustion chamber such that a tip of the injector protrudes slightly into the chamber to permit the fuel charge to be delivered thereto.
One type of fuel injector that is particularly suited for use in a direct injection engine is a so-called piezoelectric injector. Such an injector allows precise control of the timing of an injection event and of the total volume of fuel that is delivered to the combustion chamber during the injection event. This permits accurate control over the combustion process which is beneficial for fuel efficiency and exhaust emissions.
A known piezoelectric injector 2 and its associated control system 3 is shown schematically in FIG. 1. The piezoelectric injector 2 includes a piezoelectric actuator 4 that is operable to control the position of an injector valve needle 6 relative to a valve needle seat 8. As known in the art, the piezoelectric actuator 4 includes a stack 7 of piezoelectric elements that expands and contacts in dependence on the voltage across the stack 7. The axial position, or ‘lift’, of the valve needle 6 is controlled by applying a variable voltage ‘V’ to the piezoelectric actuator 4. Although not shown in FIG. 1, it should be appreciated that, in practice, the variable voltage would be applied to the actuator by connecting a power supply plug to the terminals of the injector.
Through application of an appropriate voltage across the actuator, the valve needle 6 is caused either to disengage the valve seat 8, in which case fuel is delivered into an associated combustion chamber (not shown) through a set of nozzle outlets 10, or is caused to engage the valve seat 8, in which case fuel delivery through the outlets 10 is prevented.
For further background to the invention, an injector of this type is described in applicant's European Patent No. EP0955901B. Such fuel injectors can be used in compression-ignition (diesel) engines or spark ignition (petrol) engines.
Although piezoelectric injectors are adept at delivering precise quantities of fuel with accurate timing, they also have associated disadvantages. For example, during use, a piezoelectric injector emits vibrations due to the frequency of the drive voltage that is applied to the piezoelectric actuator. The vibrations travel down the injector, or through an injector positioning/clamping arrangement, and are transmitted to the engine. The engine accentuates certain frequencies such that at least a portion of the vibrations can be detected by the human ear.
At moderate and high engine speeds, the emitted noise of the injectors is drowned out by the combustion noise of the engine. However, at low engine speeds, particularly at an engine idle operating condition and with the bonnet/hood raised, the audible injector noise is apparent. The detectable noise contributes to the overall noise/vibration/harshness (NVH) characteristics of the vehicle.
The optimisation of NVH characteristics is a significant factor in successful vehicle design since it influences the buying decision of the consumer. It is therefore desirable to reduce the amount of noise emitted by the injector in an effort to reduce the overall level of noise perceived by the user of the vehicle.